bnx2x: fix vector traveling while looking for an empty entry

Fixes the bug that may prevent from mac to be configured,
while there is an empty slot for it.

Reported-by: Maciej Żenczykowski <zenczykowski@gmail.com>
Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>